As a hollow fiber membrane type fluid treatment device including a tubular housing containing a hollow fiber membrane bundle, a hemodialyzer, hemofilter, hemodiafilter, and plasma separator used for hemodialysis, hemofiltration, and the like have been known. For example, a hemodialyzer removes wastes or toxic substances accumulated in blood based on the principle of diffusion, filtration, or the like. The hemodialyzer was put into practical use as a drum-type hemodialyzer in the middle of the twentieth century, and has been effectively still utilized for treatment of a patient suffering from partial or complete kidney failure. In general, wastes or toxic substances are removed mainly through a membrane. As the material for the membrane, regenerated cellulose and synthetic polymers such as polyacrylonitrile, polysulfone, and polyethylene have been known. As the shape of the membrane, a flat membrane and a hollow fiber membrane can be given. In recent years, a hollow fiber membrane which allows an increase in the contact area with blood and exhibits high purification performance has been widely used.
When manufacturing a hemodialyzer by using a hollow fiber membrane, several hundred to several tens of thousand hollow fiber membranes are bundled and placed in a plastic tubular housing. The housing is then filled with a potting material such as a polyurethane resin to fix the hollow fiber membranes to the housing. The resulting semifabricated product is provided with a blood introducing part (header cap) and sterilized to obtain a hemodialyzer. When treating blood by using a hemodialyzer utilizing a hollow fiber membrane, blood is caused to flow inside the hollow fiber membrane, and a dialysate containing an inorganic electrolyte or the like is caused to flow outside the hollow fiber membrane to remove wastes or toxic substances in the blood into the dialysate by utilizing the diffusion or filtration principle.
Substance removal performance is used as an index which indicates the removal performance of wastes or toxic substances in the blood of the hemodialyzer. The main factor which determines the substance removal performance is the performance, that is, the mass transfer coefficient, of the hollow fiber membrane which directly comes into contact with the blood or dialysate. Therefore, the material for the hollow fiber membrane, the size and distribution of the pores which allow a substance to pass therethrough, the thickness of the membrane which determines the permeation resistance, and the like have been studied and put in practical use.
In order to allow the hollow fiber membrane to exhibit the maximum substance removal performance, it has been mainly studied from two viewpoints of an improvement of the hollow fiber membrane bundle and an improvement of the structure and shape of the housing. For example, as for the former the optimization of the fiber density, which indicates the ratio of the cross-sectional area of the hollow fiber membrane bundle to the cross-sectional area of the tubular housing, and as for the latter the relationship between the length and the inner diameter of the housing have been studied and put in practical use. However, a conventional hemodialyzer exhibits insufficient substance removal performance, since the dialysate does not uniformly flow between the hollow fiber membranes.
In order to improve the substance removal performance and reduce the variation in the removal performance by causing the dialysate to uniformly flow through the hollow fiber membrane bundle, Japanese Patent No. 3080430, Japanese Examined Patent Publication (Kokoku) No. 59-18084, and Japanese Patent Application Laid-open No. 8-246283 disclose a technology of placing a spacer filament between the hollow fiber membranes. Furthermore, Japanese Patent Application Laid-open No. 57-194007 and WO 01/60477 disclose a technology of forming the hollow fiber membrane into a small wave shape called a crimp. These technologies aim at obtaining a uniform flow of the dialysate by creating a certain space between the hollow fiber membranes and preventing an uneven flow of the dialysate. However, in the case of forming the hollow fiber membrane bundle provided with a spacer filament, a complicated technology is required to insert the spacer filament between the hollow fiber membranes or knit the hollow fiber membrane with the spacer filament. In the case of forming the crimped hollow fiber membrane, the hollow fiber membrane breaks at the waved section or is plugged up, whereby productivity is decreased.
As to the structure and shape of the housing, Japanese Patent Application Laid-open No. 8-173527 discloses a housing shape which prevents the dialysate from remaining in the housing for a long time to obtain a uniform flow in consequence. In this technology, an inclined transition portion is provided between the end portion and the center portion of the housing, and the inclination angle continuously changes in the circumferential direction. However, since the dialysate does not uniformly flow into between the hollow fiber membranes in the vicinity of the dialysate inlet even when using this method, sufficient substance removal performance cannot be obtained. Moreover, since the space in the vicinity of the dialysate inlet of the housing is remarkably nonuniform structure, there is also a problem that a desired effect cannot be obtained when the flow rate of the dialysate is changed.
As described above, the conventional approach concerning the hollow fiber membrane and the housing shape in order to improve the substance removal performance of the hemodialyzer by making the dialysate flow uniform involves problems respectively, and the effect is not necessarily satisfactory.
In the hollow fiber membrane type fluid treatment device, if the hollow fiber membrane breaks, the treatment target liquid comes into contact with the treatment liquid, whereby a desired separation cannot be achieved. In the case where the treatment target liquid is blood, the blood flows outside the body. Therefore, the hollow fiber membrane must not be broken from the viewpoint of safety.
Various technical means have been proposed to prevent breakage of the hollow fiber membrane, particularly in the vicinity of the end portion of the hollow fiber membrane. An improvement of the hollow fiber membrane (bundle) and an improvement of the structure and shape of the housing have been mainly studied in the same manner as in the case of improving the substance removal performance.
As to a hollow fiber membrane, for example, Japanese Patent No. 3151168 and Japanese Patent Application Laid-open No. 59-4403 disclose a technology of reducing stress concentration by partially reinforcing the hollow fiber membrane. Specifically, in order to prevent occurrence of leakage by reducing both impacts due to a water stream or dropping in the hollow fiber membrane module, a resin coating layer is provided over the circumference of the hollow fiber bundle from the inner side of the resin layer on both ends of the hollow fiber membrane bundle to a position corresponding to the treatment liquid inlet and the treatment liquid outlet. However, since a very long coating layer is necessary for ensuring sufficient leakage resistance performance, the membrane area effective for substance exchange may be reduced, and the flow of the treatment liquid which flows outside the hollow fiber membrane may be affected by the provided coating layer. As a result, the removal performance of the hollow fiber membrane module may be decreased.
As to the hollow fiber membrane bundle, Japanese Patent No. 3151168 discloses a method for preventing occurrence of leakage due to breakage of the hollow fiber membrane bundle as a conventional technology by increasing the fiber density of the hollow fiber membrane bundle (i.e. the ratio of the sum of the cross-sectional areas of the hollow fiber membranes to the cross-sectional area of the tubular housing) at positions corresponding to the treatment liquid inlet and the treatment liquid outlet in the tubular housing and reducing vibration of the hollow fiber membranes due to the flow of the treatment liquid. However, as described above Japanese Patent, since it becomes very difficult to place the hollow fiber membrane bundle in the tubular housing if the fiber density is increased excessively, breakage of the hollow fiber membrane may occur when placing the hollow fiber membrane bundle in the tubular housing. Therefore, in the hollow fiber membrane module disclosed in Japanese Patent Application Laid-open No. 59-4403, the fiber density is disclosed to set in a range as low as 34 to 41%. However, since the dialysate tends to bring a short-path if the fiber density is low, this method is not necessarily preferable from the viewpoint of the substance removal performance.
A number of studies have been made about a baffle plate that is provided for preventing a water stream from directly coming into contact with the hollow fiber membrane located at the treatment liquid inlet or outlet. For example, Japanese Patent Application Laid-open No. 2000-42100 aims at preventing occurrence of breakage by increasing the space between the baffle plate and the side surface of the hollow fiber membrane bundle in the vicinity of a tongue-shaped baffle plate disposed corresponding to the treatment liquid inlet or outlet and preventing the baffle plate from directly coming into contact with the hollow fiber membrane bundle. In Japanese Patent Application Laid-open No. 2000-350781, in order to prevent the hollow fiber membrane from breaking due to adhesion with the baffle plate during manufacture (i.e. potting step), the baffle plate is formed to have such a length that the ends of the baffle plate reaches a resin layer formed on each end of the hollow fiber membrane bundle. In these technologies, the shape and size of the tongue-shaped baffle plate, formed to have a curvature almost along the inner circumferential surface in the tubular housing at an interval from the inner circumferential surface at positions corresponding to the treatment liquid inlet and outlet on the inner circumferential surface, are equal on the inlet side and the outlet side.
However, leakage due to breakage of the hollow fiber membrane also occurs due to impact caused by dropping which accidentally occurs during transportation or handling of the hollow fiber membrane module, in addition to the case where impact occurs on the hollow fiber membrane due to a water stream when the treatment liquid enters through the treatment liquid inlet or is discharged through the treatment liquid outlet. Either baffle plate as described above exhibits an excellent effect on reduction of impact due to entrance or discharge of the treatment liquid, but does not exhibit an effect of preventing occurrence of leakage due to dropping impact and the like when handling the hollow fiber membrane module. Japanese Patent Application Laid-open No. 2003-102833, for example, discloses a rim-type dispersion ring (synonymous with baffle plate) which holds the entire periphery of the hollow fiber membrane bundle and is formed along a urethane surface which is curved by centrifugal molding in order to prevent the hollow fiber membrane from breaking due to high speed water stream washing when re using the dialyzer. Such a peripheral type baffle plate is preferable from the viewpoint of protecting the entire end portion of the hollow fiber membrane bundle. However, since it is difficult to provide the baffle plate formed along the curve of the urethane surface and mold the module, this technology involves difficulty in production.
As described above, the approach concerning the hollow fiber membrane and the housing shape in order to prevent breakage of the end portion of the hollow fiber membrane has a specific problem, and the effect is not necessarily satisfactory. Therefore, the hollow fiber membrane type fluid treatment device is still unsatisfactory from the viewpoint of improvement of the substance removal performance and/or prevention of breakage of the membrane. In particular, room for improvement is still left as to the structure and shape of the housing in comparison with the approach to the hollow fiber membrane (bundle) which has been widely studied by increasing the strength or providing with the spacer, crimp, or the like.